The present invention relates to a method for surface-alloying on metal or alloy substrates, or for surface-repairing the damaged(or failed) metal or alloy substrates using laser beam. Particularly, the present invention relates to the method for surface-alloying or surface-repairing comprising the steps of: (a) plating an alloying ingredient on the surface of metal or alloy substrate, and (b) melting this plated surface using a laser beam to form an alloyed layer, the composition of which is different from the base(substrate) material. From the method of this invention, the alloyed layer, which has higher resistance to corrosion, stress corrosion cracking, fatigue, erosion and so on than that of the surface of the base material, can be formed on the surface of the substrate.
Several methods for surface-alloying have been used; thermal treatment, wet (elestroless or electro-) plating, dry plating, metalizing, thermal spraying, plasma cladding and surface alloying method using an electron beam under vacuum.
In the surface-alloying method by thermal treatment, (a) specific metal or nonmetal element(s) is(are) forced to be injected into the surface of substrate by heating the substrate and the metal or nonmetal element(s) at the same time and thus increasing the activation energy of this(these) element(s). This method includes gas or ion(plasma) carburization, nitriding (gas nitriding, nitriding in salt bath, ion nitriding), boriding, ion implantation and so on. However, these methods have limitations in some points: restriction in alloying ingredients to be implanted (e.g. restricted to nonmetal elements), restriction of the obtainable thickness of alloyed layers (e.g. several "nm" in case of ion implantation), and shape changes of the treated parts due to treatment of high temperature.
In case of wet (electroless or electron-) plating, it is difficult to obtain varieties of alloyed layers due to the restriction in alloying ingredients which can be added to the surface of a metal or alloy substrate, and in addition, the plated layer can be delaminated from the surface of the substrate.
Various alloyed layers can be formed by dry plating methods compared with the wet plating ones, but there also are some restrictions in the dry plating methods; the plating processes are rather complicated and difficult than those in other methods because of being carried out in vacuum, and it is not easy to form a thick coated layer with these methods. Furthermore, the separation of the coated layer can also occur.
Metalizing is a method that alloying processes are performed by penetrating alloying elements into base material in a salt bath at high temperature. This method is controlled by diffusion processes of the alloying ingredients into the substrate under the thermal equilibrium state conditions, which means that there may be restrictions in the kinds of alloying ingredients, and the compositions and the thickness of the alloyed layer. On the other hand, dimensional changes may be caused due to the processes at high temperature.
Using thermal spraying, various alloyed coats can be obtained. The limitations in this method, however, are pores formed in the coated layers (less than 10% in case of plasma spray, less than 5% in case of high velocity oxy-fuel spray) and oxidation of the alloying ingredients during spraying in air. In order to overcome these limitations in thermal spray techniques, spray processes under low pressure or in vacuum have been developed. But even with these new methods, it is nearly impossible to obtain perfect pore free coated layers. Moreover delamination on the coated layer formed by the thermal spray may occur in use since the coated layer is mechanically bound with the base material.
Plasma cladding method is now under development, which is a much more flexible process to obtain a desired alloyed layer and its thickness in air. But heat affected zone with this method is larger than that with the process with a laser beam is, since the energy density of the plasma is lower than that of a laser beam. A limitation of this preocess is difficult to form a uniform surface-alloyed layer of a part having geometrically complicated shape.
A surface alloying method using an electron beam has restrictions in the size and shape of the treated part because the whole process should be carried out under vacuum and electron gun cannot move freely.
The present inventors have successfully completed surface alloying on Ni-base alloy with a laser beam, the method of which is superior to the former methods described above.